[filter-effects] Tainted filter primitives

http://dev.w3.org/fxtf/filters/#security

For feFlood, feDropShadow, feDiffuseLighting and feSpecularLighting, I
don't think these should be tainted --- currentColor isn't used very often.
In Gecko (and I think other engines), we make getComputedStyle on 'color'
return the value the 'color' property would have if all links are
unvisited. So I think we can use that here, and specify that for filter
primitive elements, currentColor evaluates to the value of the 'color'
property assuming no links are visited.

feImage is only tainted if the mode is No-CORS and the loaded image
actually is from a different origin.

Rob
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